Asphalt pavements basically consist of two main components; bitumen and aggregate. The function of bitumen is to act as a binder in between the aggregate skeleton, giving the asphalt sufficient internal cohesion. It is therefore of vital importance that the bitumen has a strong bond (adhesion) to the aggregate surface.
The fact that roadways can suffer water damage is well established. The visible symptoms of water damage are various and include rutting and shoving, deformation, loss of chippings from surface dressings (chip seals), and raveling of surface layers. This leads to rough surfaces and eventually potholes, loss of structural strength, susceptibility to freeze-thaw damage and cracking. The underlying problem on a micro scale is loss of adhesion between the bitumen and the aggregate surface. Even though the aggregate is fully coated with bitumen, water could penetrate the bitumen film by various means, as for instance through wearing of thin bitumen films at sharp aggregate edges.
In common terms, bitumen is an oily material and therefore very hydrophobic. Bitumen has much less affinity for the aggregate surface than water has, and it does not adhere easily to the hydrophilic surfaces of most aggregates. The adhesion between bitumen and aggregate depends on the chemical nature of the components, and therefore the source of the bitumen and type of aggregate. Aggregate properties such as surface texture, porosity, shape and absorption will also influence the aggregate/bitumen adhesion.
Due to its chemical composition, bitumen has quite a low polarity whereas water is extremely polar. Aggregates may be of an “acidic” type, with surfaces that tend to be negatively charged, or “basic” (also referred to as “alkaline”)” with surfaces that tend to be positively charged. Acidic aggregates include those with high silica contents, while basic aggregates include carbonates.
It is the function of the adhesion promoter to alter the relative surface properties and polarity of incompatible materials, thus facilitating a strong bond between the bitumen and the aggregate, which resists the water displacing effects for the service life of the pavement.
The adhesion could either be passive or active. Passive adhesion is the ability of a binder (bitumen) to maintain the integrity of the adhesive bond with aggregate to prevent stripping under wet conditions, which can be assured by the addition of an adhesion promoter to the binder. Active adhesion is achieved by the action of the adhesion promoter to decrease the contact angle of the bitumen to aggregate interface, thus allowing the bitumen to displace water and coat the aggregate surface.
There are earlier publications describing the use of phosphated alcohols as adhesion improvers.
EP 0 926 191 relates to an asphalt additive which increases the adhesion between bitumen and aggregates. The asphalt additive comprises at least one phosphated product derived from a monohydric alcohol having either a linear or methyl branched hydrocarbon radical, and which is optionally alkoxylated.
EP 0 157 210 relates to a method for strengthening the adhesion between heated bitumen and aggregates by adding to the bitumen an acidic organophosphorous compound exemplified by e.g. monooleyl phosphate, dioleyl phosphate and phosphate of sorbitan laurate.
FR 2 971 785 relates to the use of a composition comprising at least one phospholipid and at least one salt of a fatty acid to produce a bituminous product, where the bituminous product is an emulsion comprising 0.2-10 wt % of the said composition, 1-80 wt % bitumen and water up to 100 wt %.
However, there is still a need for adhesion promoters having an increased efficiency towards a wide range of aggregates.